284 Methods in Plant Histology 



It is absolutely necessary to note the tube length, length of the bar 

 of the camera mirror and inclination of the camera mirror, and the 

 level at which the scale is made. A variation in any of these details 

 will change the scale. 



In using the stage micrometer, place the cardboard on the table, 

 and with the aid of the camera lucida sketch the rulings of the microm- 

 eter. In Fig. 104 (which has been reduced by photography) note, 

 for example, the scale drawn for Leitz objective 3. The spaces are 

 drawn from the tenths of a millimeter rulings of the stage microm- 

 eter. Therefore each space on the card represents one-tenth of a 

 millimeter, or 100 /j, and the ten spaces shown on the card represent 

 1 mm., or 1,000 /j. By measuring with a metric rule the ten spaces 

 upon the card, it is found that the scale is 114 mm. in length. The 

 magnification of any drawing made with the same ocular and objec- 

 tive, under the same conditions, will therefore be 114 diameters. 

 This does not mean that the magnifying power of Leitz objective 3 

 with Zeiss ocular 4 is 114 diameters, for the magnification of this 

 combination is much less. A scale drawn at the level of the stage 

 would show more nearly the magnifying power of the combination, 

 but would still give too large a figure. The exact size of any object 

 which has been sketched with this combination can now be measured 

 by applying the cardboard scale, just as one would measure gross 

 objects with a rule. 



The diameter of the field with this combination is 1,800/z. By 

 knowing the diameter of the field with the various combinations, one 

 can guess approximately the size of objects. 



Other combinations are made in the same way. An ex- 

 cellent check on the accuracy of the computations is to measure 

 the same object by means of the ocular micrometer and by the 

 scale card. If the results are the same, the computations are 

 correct. 



In making sketches, it is a good plan to add the data which would 

 be needed at any time in making measurements; e.g., L. 3, Z. oc. 4, 

 table, 110, 50, would show that the sketch was made with Leitz 

 objective 3, Zeiss ocular 4, at the level of the table, with mirror bar 

 at 110, and camera mirror at 50. 



